超市进销存管理系统-外文翻译.docx

超市进销存管理系统-外文翻译.docx

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外文原文

ThebusyJavadeveloper'sguidetoScala:

Classaction

ItmakessenseforJava?

developerstouseobjectsasafirstpointofreferenceforunderstandingScala.InthissecondinstallmentofThebusyJavadeveloper'sguidetoScalaseries,TedNewardfollowsabasicpremiseoflanguagemeasurement:

thatthepowerofalanguagecanbemeasuredindirectrelationtoitsabilitytointegratenewfacilities--inthiscase,supportforcomplexnumbers.Alongthewayyou'llseesomeinterestingtidbitsrelatedtoclassdefinitionsandusageinScala.Inlastmonth'sarticle,yousawjustatouchofScala'ssyntax,thebareminimumnecessarytorunaScalaprogramandobservesomeofitssimplerfeatures.TheHelloWorldandTimerexamplesfromthatarticleletyouseeScala'sApplicationclass,itssyntaxformethoddefinitionsandanonymousfunctions,justaglimpseofanArray[],andabitontype-inferencing.Scalahasagreatdealmoretooffer,sothisarticleinvestigatestheintricaciesofScalacoding.

Scala'sfunctionalprogrammingfeaturesarecompelling,butthey'renottheonlyreasonJavadevelopersshouldbeinterestedinthelanguage.Infact,Scalablendsfunctionalconceptsandobjectorientation.InordertolettheJava-cum-Scalaprogrammerfeelmoreathome,itmakessensetolookatScala'sobjectfeaturesandseehowtheymapovertoJavalinguistically.Bearinmindthatthereisn'tadirectmappingforsomeofthesefeatures,orinsomecases,the"mapping"ismoreofananalogthanadirectparallel.Butwherethedistinctionisimportant,I'llpointitout.

Scalahasclass（es）,too

RatherthanembarkonalengthyandabstractdiscussionoftheclassfeaturesthatScalasupports,let'slookatadefinitionforaclassthatmightbeusedtobringrationalnumbersupporttotheScalaplatform（largelyswipedfrom"ScalaByExample"--seeResources）:

Listing1.rational.scala

classRational（n:

Int,d:

Int）

{

privatedefgcd（x:

Int,y:

Int）:

Int=

{

if（x==0）y

elseif（x<0）gcd（-x,y）

elseif（y<0）-gcd（x,-y）

elsegcd（y%x,x）

}

privatevalg=gcd（n,d）

valnumer:

Int=n/g

valdenom:

Int=d/g

def+（that:

Rational）=

newRational（numer*that.denom+that.numer*denom,denom*that.denom）

def-（that:

Rational）=

newRational（numer*that.denom-that.numer*denom,denom*that.denom）

def*（that:

Rational）=

newRational（numer*that.numer,denom*that.denom）

def/（that:

Rational）=

newRational（numer*that.denom,denom*that.numer）

overridedeftoString（）=

"Rational:

["+numer+"/"+denom+"]"

}

WhiletheoverallstructureofListing1islexicallysimilartowhatyou'veseeninJavacodeoverthelastdecade,somenewelementsclearlyareatworkhere.Beforepickingthisdefinitionapart,takealookatthecodetoexercisethenewRationalclass:

Listing2.RunRational

classRational（n:

Int,d:

Int）

{

//...asbefore

}

objectRunRationalextendsApplication

{

valr1=newRational（1,3）

valr2=newRational（2,5）

valr3=r1-r2

valr4=r1+r2

Console.println（"r1="+r1）

Console.println（"r2="+r2）

Console.println（"r3=r1-r2="+r3）

Console.println（"r4=r1+r2="+r4）

}

WhatyouseeinListing2isn'tterriblyexciting:

Icreateacoupleofrationalnumbers,createtwomoreRationalsastheadditionandsubtractionofthefirsttwo,andechoeverythingtotheconsole.（NotethatConsole.println（）comesfromtheScalacorelibrary,livinginscala.*,andisimplicitlyimportedintoeveryScalaprogram,justasjava.langisinJavaprogramming.）

HowmanywaysshallIconstructthee?

NowlookagainatthefirstlineintheRationalclassdefinition:

Listing3.Scala'sdefaultconstructor

classRational（n:

Int,d:

Int）

{

//...

Althoughyoumightthinkyou'relookingatsomekindofgenerics-likesyntaxinListing3,it'sactuallythedefaultandpreferredconstructorfortheRationalclass:

nanddaresimplytheparameterstothatconstructor.

Scala'spreferenceforasingleconstructormakesacertainkindofsense--mostclassesenduphavingasingleconstructororacollectionofconstructorsthatall"chain"throughasingleconstructorasaconvenience.Ifyouwantedto,youcoulddefinemoreconstructorsonaRationallikeso:

Listing4.Achainofconstructors

classRational（n:

Int,d:

Int）

{

defthis（d:

Int）={this（0,d）}

NotethatScala'sconstructorchaindoestheusualJava-constructor-chainingthingbycallingintothepreferredconstructor（theInt,Intversion）.

Details,（implementation）details...

Whenworkingwithrationalnumbers,ithelpstoperformabitofnumericallegerdemain:

namelythatoffindingacommondenominatortomakecertainoperationseasier.Ifyouwanttoadd1-over-2（alsoknownas"one-half"）to2-over-4（alsoknownas"two-fourths"）,theRationalclassshouldbesmartenoughtorealizethat2-over-4isthesameas1-over-2,andconvertitaccordinglybeforeaddingthetwotogether.

Thisisthepurposeofthenestedprivategcd（）functionandgvalueinsideoftheRationalclass.WhentheconstructorisinvokedinScala,theentirebodyoftheclassisevaluated,whichmeansgwillbeinitializedwiththegreatestcommondenominatorofnandd,andthenusedinturntosetnanddappropriately.

LookingbackatListing1,it'salsofairlyeasytoseethatIcreatedanoverriddentoStringmethodtoreturnthevaluesofRational,whichwillbeveryusefulwhenIstartexercisingitfromtheRunRationaldrivercode.

NoticethesyntaxaroundtoString,however:

theoverridekeywordinthefrontofthedefinitionisrequiredsothatScalacanchecktomakesurethatacorrespondingdefinitionexistsinthebaseclass.Thiscanhelppreventsubtlebugscreatedbyaccidentalkeyboardslips.（Itwasthissamemotivationthatledtothecreationofthe@OverrideannotationinJava5.）Notice,aswell,thatthereturntypeisnotspecified--it'sobviousfromthedefinitionofthemethodbody--andthatthereturnedvalueisn'texplicitlydenotedusingthereturnkeyword,whichJavawouldrequire.Instead,thelastvalueinthefunctionisconsideredthereturnvalueimplicitly.（YoucanalwaysusereturnkeywordifyoupreferJavasyntax,however.）

Somecorevalues

Nextuparethedefinitionsofnumeranddenom,respectively.Thesyntaxinvolved,offhand,wouldleadtheJavaprogrammertobelievethatnumeranddenomarepublicIntfieldsthatareinitializedtothevalueofn-over-gandd-over-g,respectively;butthisassumptionisincorrect.

Formally,Scalacallsnumeranddenommethodswithoutparameters,whichareusedtocreateaquick-and-easysyntaxfordefiningaccessors.TheRationalclassstillhasthreeprivatefields,n,d,andg,buttheyarehiddenfromtheworldbydefaultprivateaccessinthecaseofnandd,andbyexplicitprivateaccessinthecaseofg.

TheJavaprogrammerinyouisprobablyaskingatthispoint,"Wherearethecorresponding"setters"fornandd?

"Nosuchsettersexist.PartofthepowerofScalaisthatitencouragesdeveloperstocreateimmutableobjectsbydefault.Granted,syntaxisavailabletocreatemethodsformodifyingtheinternalsofRational,butdoingsowouldruintheimplicitthread-safenatureofthisclass.Asaresult,atleastforthisexample,I'mgoingtoleaveRationalasitis.

Naturally,thatraisesthequestionofhowonemanipulatesaRational.Likejava.lang.Strings,youcan'ttakeanexistingRationalandmodifyitsvalues,sotheonlyalternativeistocreatenewRationalsoutofthevaluesofanexistingone,orcreateitfromscratch.Thisbringsintofocusthenextsetoffourmethods:

thecuriouslynamed+,-,*,and/methods.

Andno,contrarytowhatitmightlooklike,thisisn'toperator-overloading.

Operator,ringmeanumber

RememberthatinScalaeverythingisanobject.Inthelastarticle,yousawhowthatprincipleappliestotheideathatfunctionsthemselvesareobjects,whichallowsScalaprogrammerstoassignfunctionstovariables,passfunctionsasobjectparameters,andsoon.Anequallyimportantprincipleisthateverythingisafunction;thatistosay,inthisparticularcase,thereisnodistinctionbetweenafunctionnamedaddandafunctionnamed+.InScala,alloperatorsarefunctionsonaclass.Theyjusthappentohave,well,funkynames.

IntheRationalclass,then,fouroperationshavebeendefinedforrationalnumbers.Thesearethecanonical,mathematicaloperationsadd,subtract,multiply,anddivide.Eachoftheseisnamedbyitsmathematicalsymbol:

+,-,*,and/.

Notice,however,thateachoftheseoperatorsworksbyconstructinganewRationalobjecteachtime.Again,thisisverysimilartohowjava.lang.Stringworks,anditisthedefaultimplementationbecauseityieldsthread-safecode.（Ifnosharedstate--andinternalstateofanobjectsharedacrossthreadsisimplicitlysharedstate--ismodifiedbyathread,thenthereisnoconcernoverconcurrentaccesstothatstate.）

What'snewwithyou?

Theeverythingisafunctionrulehastwopowerfuleffects:

Thefirst,asyou'vealreadyseen,isthatfunctionscanbemanipulatedandstoredasobjectsthemselves.Thisleadstopowerfulre-usescenariosliketheoneexploredinthefirstarticleinthisseries.

ThesecondeffectisthatthereisnospecialdistinctionbetweentheoperatorsthattheScala-languagedesignersmightthinktoprovideandtheoperatorsthatScalaprogrammersthinkshouldbeprovided.Forexample,let'sassumeforamomentthatitmakessensetoprovidean"inversion"operator,whichwillflipthenumeratoranddenominatorandreturnanewRational（sothatRational（2,5）willreturnRational（5,2））.Ifyoudecidethatthe~symbolbestrepresentsthisconcept,thenyoucandefineanewmethodusing